The CNS is an important reservoir for HIV, but we do not know why virus accumulates in the brain, nor how it leaves to re-seed the periphery. We have found that patients whose peripheral blood cells migrate across endothelial barriers carrying replication-competent virus, have a worse prognosis as defined by increased viral loads, more hospitalizations for AIDS-related illnesses, and death. The distinguishing feature is not just the presence of circulating provirus-positive cells, but the fact that these patients' leukocytes migrate spontaneously and carry infectious virus across vascular barriers. These HIV-infected lymphocytes establish subendothelial foci of infected cells. Uninfected leukocytes migrating into these foci reverse-migrate back across confluent endothelial monolayers carrying infectious virus with them. We postulate that trafficking of infected MNLs in and out of subendothelial leukocyte depots increases viral burden and helps to accelerate progression of disease. To test this hypothesis we propose the following: In specific Aim #1 we will identify the cellular interactions and molecular signals that regulate the reverse-migration of cells carrying replication-competent virus through cerebrovascular vs. peripheral vascular barriers in vitro. We postulate that the cellular microenvironment within the brain facilitates the development of these foci and explains why the brain can become a reservoir that disseminates HIV- I to other sites. We will determine whether astrocytes and/or microglia accelerate transendothelial migration of infected leukocytes and their accumulation below endothelial barriers. We will also investigate the role these cells play in causing infected T cells and monocyte-derived dendritic cells to migrate back out of these subendothelial foci into the vascular lumen. In specific Aim #2 we will identify the effects of inflammatory stimuli on the reverse-migration of cells carving replication-competent virus. We postulate that encounters with soluble or cell associated immune complexes, cell binding fibronectin fragments, and/or TNF-OL enhance formation of subendothelial infiltrates containing infected cells and the generation of reverse-migratory cells that carry replication-competent virus. In specific Aim #3 we will test the hypothesis that HIV infections accelerate particularly rapidly in patients whose leukocytes form subendothelial infiltrates of infected cells that generate large numbers of reverse-migratory leukocytes that can disseminate virus to distal tissue sites.